An insulated gate semiconductor device such as a trench gate type transistor and an IGBT functions as a high break down voltage insulated gate semiconductor device having a trench gate structure. FIGS. 17 and 18 show IGBTs disclosed in JP-A-2006-49455. Each IGBT includes an N+ type emitter region 101 for contacting an emitter electrode is selectively formed in a P type base region 102. A dummy trench 103 is formed in a region other than the emitter region 101. Thus, multiple trenches are homogeneously formed. Specifically, the emitter region 101 is not formed in a whole base region 102, but formed in a part of the base region 102. The trench 105 is formed in the part of the base region 102. A gate electrode 104 is formed in the trench 105 so that a gate voltage is applied to the gate electrode 104. No emitter region 102 is formed in another part of the base region 102. However, a dummy trench 103 is formed in the other part of the base region 102. A dummy gate electrode 106 is formed in the dummy trench 103.
Thus, the emitter region 101 is selectively formed in the base region 102, so that conductivity modulation in the base region 102 is promoted. Here, the base region 102 has high resistance. Accordingly, energization loss is much reduced. Since the dummy trench 103 is formed, a break down voltage is improved. Both of the energization loss and the break down voltage are improved. In the IGBT, to stabilize an electric potential of the dummy gate electrode 106, the dummy gate electrode 106 is connected to the emitter electrode E, as shown in FIG. 17. Alternatively, the dummy gate electrode 106 may be connected to the gate electrode 104, as shown in FIG. 18.
However, when the dummy gate electrode 106 in the dummy trench 103 is connected to the emitter electrode E or the gate electrode 104, the following difficulties arise.
When the dummy gate electrode 106 is connected to the gate electrode 104, a capacitance between the gate G and the collector C increases, so that a switching loss becomes large. Further, when the dummy gate electrode 106 is connected to the emitter electrode E, a capacitance between the gate G and the emitter E increases, so that a switching surge voltage becomes large.
Further, a trench gate IGBT among a power semiconductor device operates with a MOS gate driving method, so that controllability of the device is very high. Further, a bipolar operation is performed in the IGBT, and thereby, a saturated voltage is comparatively low. Accordingly, the IGBT is used for many applications. Since the power device is used as a non-contact switch, it is preferred to have a small generation loss. It is required for the IGBT to have small saturated voltage and a low switching loss. A relationship between the saturated voltage of the IGBT and a switching loss, i.e., a turn-off loss of the IGBT is trade-off. In general, the relationship of the trade off represents a trade off characteristics, which shows an index of generated loss in the power device. Thus, it is required to improve the trade off characteristics. Further, it is also required for the device to reduce an electro magnetic noise. To reduce the electro magnetic noise, it is necessary to reduce a voltage drop speed (i.e., dV/dt) and a current increase acceleration (i.e., dIc/dt) in case of turning off. However, when the dV/dt and dIc/dt are reduced, the switching loss increases. Thus, it is difficult to reduce both of the electro magnetic noise and the switching loss. In general, the turn-on loss and the electro magnetic noise have a trade off relationship. Here, to reduce the electro magnetic noise, it is important to change a hard switching to a soft switching when the device turns on without increasing the turn-on loss. This is, the waveform of dIc/dt is changed from large to small.
Regarding the electro magnetic noise when the IGBT turns on, it is known that a device characteristic much affects the electro magnetic noise in a case where the IGBT turns on with a small current equal to one-tenth of a current rating. Specifically, the reason why the electro magnetic noise having a frequency in a range equal to or larger than 30 MHz is generated may relate to high voltage drop speed having a high frequency component. Accordingly, to maintain the dvdt in case of switching within a threshold at which the electro magnetic noise is not generated, a gate resistance is controlled so that a main current increase rate (i.e., dIc/dt) in case of turning on is limited.
When only the gate resistance increases, the turn-on loss of the IGBT increases when the IGBT turns on. Thus, when the gate resistance increases, the current increase rate in case of turning on is reduced, and a voltage tail also increases. Thus, the switching loss increases. Accordingly, in the characteristics of the trench type IGBT, it is preferable that the gate resistance is comparatively small, and the dIc/dt is sufficiently small.
The trench type IGBT is shown in FIG. 31. The N channel type IGBT includes a trench gate structure having a stripe pattern on a surface of the silicon substrate. FIG. 31 is a cross sectional view showing the IGBT along with a direction in parallel to the stripe pattern and perpendicular to the silicon substrate. In FIG. 31, the IGBT includes an N type base layer 201 having a small impurity concentration, a P type collector layer 202 having a high impurity concentration and disposed on a principal surface of the base layer 201, and a P type channel region 203a disposed on another principal surface of the base layer 201. A N+ type emitter region 204 is selectively formed in a surface portion of the channel region 3a. A trench 205a is formed from a surface on an emitter region side and penetrates the channel region 203a and reaches the base layer 201. A gate electrode 207a is formed in the trench 205 through an insulation film 206. The gate electrode 207a is made of conductive poly silicon. A method for forming the device is such that the trench 205a is formed on the surface of the channel region 203a, the gate insulation film is formed in the trench 205a, and the gate electrode 207a is filled in the trench 205a, and then, the emitter region 204 is formed. Further, an interlayer insulation film 20 is formed to cover the gate electrode 207a. Furthermore, the emitter electrode 210 made of a metallic film is formed over the interlayer insulation film 208. The emitter electrode 210 contacts the surface of the emitter region 204 and the surface of the channel region 203a. In general, the trench type IGBT includes a P type body region 209 having a high impurity concentration and disposed on a part of the surface of the channel region 203a so that a latch-up immunity is improved. A N type buffer region or a field stop region 211 having a middle impurity concentration is formed between the base layer 201 and the collector layer 202. The buffer region or the field stop region 211 is made of Se. Further, a protection film may be formed over the silicon substrate. The protection film is made of a silicon nitride film, an amorphous silicon film or a poly imide film. A collector electrode 220 made of a metallic film is formed on the surface of the collector layer 202.
The operation of the IGBT for turning on will be explained. When the IGBT turns off, the emitter electrode 210 is grounded, and a voltage is applied to the collector electrode 220, so that the IGBT shows a blocking state when a voltage is smaller than an inverse breakdown voltage since the base layer 201 and the channel region 203a provides an inverse bias PN junction. In this case, when a voltage higher than a threshold voltage is applied to the gate electrode 207a, a charge is accumulated in the gate electrode 207a from a gate driving circuit through the gate resistance. At the same time, an N type channel switched from a P type to an N type is formed in a surface portion of the channel region 203a along with a trench sidewall. The N type channel is arranged between the emitter region 204 and the base layer 201, the emitter region 204 is exposed on the sidewall of the trench 205a. The channel region 203a contacts the gate electrode 207a through the gate insulation film 206. When the N type channel is formed, the inverse bias junction is disappeared in the N type channel, so that the electrons are injected in the base layer 201 through the emitter electrode 210, the emitter region 204 and the N type channel in the channel region 203a. When the electrons are injected in the base layer 201, a forward voltage is applied to the PN junction between the collector layer 202 and the base layer 201, so that holes as a minor carrier is injected in the base layer 201 from the collector layer 202. When the holes are injected in the base layer 201, the electron concentration of the electrons as a major carrier increases to maintain neutrality of the carriers in the base layer 201, and thereby, the resistance of the base layer 201 is reduced. Here, this phenomenon is called a conductivity modulation. In this case, if the voltage drop caused by the current flowing between the collector electrode 220 and the emitter electrode 210 is substantially equal to the on state voltage of the diode formed between the collector layer 202 and the emitter region 204, the on-state voltage of the IGBT shows an ideal voltage.
Next, to switch the IGBT from an on-state to an off-state, a voltage between the emitter electrode 210 and the gate electrode 207a is reduced to be smaller than the threshold. At this time, the charge accumulated in the gate electrode 207a is discharged to the gate driving circuit through the gate resistor, and the channel inverted to the N type is switched to the P type so that no channel exists in the IGBT. Thus, the electron supply is stopped, and the hole injection from the collector layer 202 is also stopped. However, the current continues to flow until the electrons in the base layer 201 is completely transmitted to the collector electrode 220, and the holes in the base layer 201 completely is transmitted to the emitter electrode 210, or until the electrons and the holes are completely re-combined so that they are disappeared. After the accumulated electrons and the accumulated holes are disappeared, the current stops flowing.
In the trench type IGBT, to reduce the on-state resistance, various methods are performed. For example, an IEGT (i.e., injection enhanced gate bipolar transistor) has characteristics, which are most excellent and close to a maximum limit of the on-state voltage of the diode. In the IEGT, as shown in FIG. 12, a part of the principal surface of the emitter region 204 and a part of the principal surface of the channel region 203a in the cell are covered with the interlayer insulation layer 208, so that these regions 203a, 208 do not contact the emitter electrode. The operation of the IEGT is similar to the trench type IGBT. The part of the emitter region 204 and the part of the channel region 203a not contacting the emitter electrode 210, and the holes in a portion under the P type channel region 203a are not discharged to the emitter electrode 210, so that the holes are accumulated in the portion. Thus, the carrier concentration distribution of the base layer 201 becomes closer to the carrier concentration distribution of the diode. Thus, the on-state voltage of the IEGT is smaller than that of the IGBT (which is disclosed in JP-A-H05-243561). Further, to improve both of the on-state voltage and the switching characteristics, a trench type IGBT is disclosed in JP-A-2000-228519, in which the accumulated carrier concentration on the emitter electrode side is increased.
It is required for the power device to have a low on-state voltage and high-speed switching characteristics. It is also required to improve the on-state voltage and the switching characteristics. However, the trench structure is formed to have high density in the trench type IGST and the IEGT so that they have a low on-state voltage. Accordingly, the capacity between the gate electrode and the emitter electrode increases, so that the switching characteristics are reduced. Accordingly, the switching loss increases. In this case, the low on-state voltage and the low switching loss or the high-speed switching characteristics are related to trade off relationship. Thus, it is difficult to improve both of the on-state voltage and the switching characteristics.
Further, in general, the waveform of the device having the high-speed switching characteristics is hard when the device switches so that the device has a hard switching characteristics. An electro magnetic noise is easily generated in the device. Thus, it is difficult to form the power device such as IGBT having soft switching characteristics so that the device shows a soft waveform to reduce the electro magnetic noise and to have high-speed switching characteristics.
As described above, when the IGBT switches from the on-state to the off-state, it is necessary to charge and discharge the capacitance between the gate electrode and the emitter electrode. When the capacitance is large, the charge and discharge time increases so that the loss increases. Further, it is necessary to have a large gate driving circuit. The loss in the power device is a sum of a stationary loss defined by the on-state voltage and a switching loss in case of turning on and off. It is important to reduce the on-state voltage and to reduce the switching loss, i.e., to reduce the capacitance between the gate electrode and the emitter electrode. A semiconductor device having a low on-state voltage and a low capacitance between the gate electrode and the emitter electrode is shown in FIG. 29. The device has a P type channel region with a P type region without an N+ type emitter region. Further, the P type region is not connected to the emitter electrode, so that the P type region is isolated. The P type region is in a floating state. The device is a trench type IEGT, which is disclosed in JP-A-2001-308327. Further, a trench type semiconductor device for a power source is disclosed in JP-A-H09-139510. This device has a low on-state voltage and a low stationary loss. Furthermore, FIG. 33 shows a trench type semiconductor device having a low on-state voltage, a low capacitance between a gate electrode and an emitter electrode and a high break down voltage. This device is disclosed in JP-A-2003-188382 and JP-A-2006-49455. FIGS. 30 and 34 also show trench type semiconductor device as a comparison having a low on-state voltage, a low capacitance between a gate electrode and an emitter electrode and a high break down voltage, according to a related art.
However, in the trench type IGBT shown in FIGS. 29-34 and the trench type IGBT and the IEGT disclosed in the above references, the break down voltage may be low. Alternatively, since these devices have the hard switching characteristics, electro magnetic noise is easily generated in the devices. The reason why it is difficult to increase break down voltage in the IGBT and the IEGT is such that an electric field distribution in a silicon substrate is inhomogeneous when the device turns off (i.e., when a voltage is not applied to the device). Thus, the electric field is concentrated at a bottom of the trench gate, so that the device may be broken down at a voltage lower than a specification break down voltage. The reason why the devices have the hard switching characteristics is such that a ratio between the capacitance between the gate and the collector and the capacitance between the gate and the emitter is small.
Thus, it is required to reduce the on-state voltage to be equal to that of the IEGT and to reduce the switching loss and to have the high break down voltage. Further, it is required to have soft switching characteristics.